Donald Trump began celebrating Super Tuesday early at a rally in Valdosta, Georgia. “Tomorrow is a big day and I want you to lead the pack,” he told the crowd at Valdosta State University. “We are going to have a ... more >

ATLANTA — The GOP presidential field mounted a last-ditch effort to derail Donald Trump ahead of Super Tuesday’s slate of primaries, with Sen. Marco Rubio demanding the billionaire businessman begin making his signature clothing lines in the U.S., while Sen. Ted Cruz accused him of plotting to go back on his vow to deport illegal immigrants.

Stunned by polls showing Mr. Trump easily leading in most of the dozen states that vote Tuesday — and a new poll showing him approaching majority support among Republicans nationwide — Republican leaders, strategists and candidates pleaded with voters to back away from Mr. Trump.

But Mr. Trump is also starting to build momentum among some of those leaders, having earned the support in the last week of six governors or members of Congress, as well as other lower-ticket state officials who say his stance on immigration makes him the only viable candidate for the GOP.

“Now, more than ever, America needs Mr. Trump’s aggressive approach to the problem of illegal immigration,” Kansas Secretary of State Kris Kobach, the architect of many of the state-level immigration crackdown proposals in recent years, said in a statement adding himself to Mr. Trump’s list of supporters.

Voters also continue to build support behind Mr. Trump, with a CNN/ORC survey of registered Republicans and GOP-leaners showing him with 49 percent of the vote nationally. That suggests that Mr. Trump was right in saying that he would pick up supporters when other candidates withdrew from the race, such as New Jersey Gov. Chris Christie and former Florida Gov. Jeb Bush.

“Tomorrow is a big day and I want you to lead the pack,” he told the crowd at Valdosta State University. “We are going to have a lot of success.”

Hitting on familiar campaign themes, the real estate mogul and reality TV star lamented that the U.S. does not win in trade, at the border, in education or in fighting ISIS. And he vowed as president to reverse that losing streak.

“You are going to look back and you’re going to say that was an evening when we really became great again,” said Mr. Trump, his voice rising. “We are going to win at every single level. We are going to win at the borders. We are going to win with our military. We are going to win, win, win. We are going to make America great again — greater than ever before.”

Mr. Rubio and Mr. Cruz were desperate to find a hole in Mr. Trump’s armor, and took aim at his populist positions on immigration and trade, telling voters the businessman is trying to defraud voters.

Mr. Cruz, joined later in the day by Mr. Rubio, demanded Mr. Trump call The New York Times editorial board and get it to release the tape or transcript of an off-the-record session they held in January. According to a report on BuzzFeed.com, Mr. Trump may have told the paper he’s willing to be more flexible on his strict immigration stance.

“I call on Donald: Ask The New York Times to release the tape,” Mr. Cruz told reporters. “And do so today, before the Super Tuesday primary.”

The paper told BuzzFeed it would decide what to do if Mr. Trump did call for its release.

Mr. Rubio and Mr. Cruz are battling to be the candidate the anti-Trump forces rally behind. But neither has shown any inclination to withdraw and create the one-on-one matchup that could derail Mr. Trump.

“There is only one campaign that has beaten, and that can beat, Donald Trump,” Mr. Cruz said again ahead of a Texas rally on Tuesday.

Mr. Rubio made a last-ditch appeal at a rally in Atlanta, warning that Mr. Trump was conning Republican voters and that he could not win the White House in a general election.

“A vote for Donald Trump tomorrow is a vote for Hillary Clinton in November,” said Mr. Rubio, his voice hoarse from nonstop campaigning.

“I will go to all 50 states and every territory. I will continue to speak out until I literally have no voice left,” he said. “I will go anywhere and speak to anyone before I will let a con artist get ahold of the Republican nomination.”

The crowd responded enthusiastically. And yet, their fervor was dampened by the daunting task confronting their candidate, who trails in Georgia and every other Super Tuesday state.

The Rubio campaign is hoping he can place second in most of the contests, giving him a reason to fight on to Florida.

“I just keep hoping,” said Rubio supporter Jenna Tagliaferri, 49. “I hope he doesn’t give up and Cruz will drop out first.”

Mr. Trump, meanwhile, continues to fan the flames of the civil war within the GOP.

Evangelical leaders begged their flock not to back him, with the Christian Post penning an editorial calling the businessman “misogynist and a philanderer.” But Mr. Trump continues to do very well among evangelical voters.

At the same time, Mr. Trump is holding his own among moderate GOP voters, and has been helped by defending the non-abortion services provided by Planned Parenthood, according to Democracy Corps, a Democratic firm.

Mr. Greenberg said the divisions within the GOP are deep, and could explode in a disastrous way for Republicans come November.

Sen. Ben Sasse, Nebraska Republican, underscored the potential split when he announced he would not be able to support Mr. Trump as the GOP’s nominee. In a battle between Mr. Trump and Hillary Clinton, Mr. Sasse said he’d find a third candidate.

At the same time, Mr. Trump has begun to win the backing of other Republicans. After entering last week without a single governor or member of Congress supporting him, he now has three House members, Govs. Chris Christie of New Jersey and Paul LePage of Maine, and Sen. Jeff Sessions backing his campaign.

The endorsements of Mr. Sessions and Mr. Kobach could be particularly important, joining that of Maricopa County, Arizona, Sheriff Joe Arpaio. Together the three represent the chief faces of the push to crack down illegal immigration at the federal state and local levels.

Mr. Kobach said Mr. Trump “stands head and shoulders above the other candidates” when it comes to toughness on illegal immigration.

He even said he had thoughts for ways Mr. Trump could live up to his pledge to make Mexico pay for the border wall, including using a section of the USA Patriot Act to dock some of the billions of dollars in remittances Mexicans working in the U.S. send back home to Mexico every year.